Studies were performed to investigate the development of specific macrophage functions after differential in vitro activation and to identify cell surface markers of macrophages that could be used to identify macrophages at different stages of activation and/or differentiation. Macrophages can be differentially activated in vitro to express suppressor or antitumor functions. Cytolytic and cytostatic activities can be induced by lymphokines (LK) plus either low dose endotoxin (LPS) or by high dose LPS. However, low dose LPS will induce cytostatic, but not cytolytic function. Natural suppressor activity, as demonstrated by inhibition of lymphoproliferation, is expressed in resident and peptone-elicited macrophages. In vitro activation with LK + low dose LPS or high dose LPS reverses the suppressive effect. Two interrelated projects were pursued which examined cell surface markers associated with macrophages at different stages of activation. We found that the neutral glycolipid, asialo-GM1, is found on all macrophage populations but in varying amounts. There is an increase in asialo GM1 (asGM1) in macrophages that have been activated to become tumoricidal as indicated by 1) enhanced fluorescence on activated cell populations, 2) decreased fluorescence intensity and cytolytic activity after in vivo treatment with antisera to asGM1, and 3) positive correlation between high expression of asGM1 and tumoricidal activity after the cells were sorted for this marker. Other stimuli such as thioglycollate and agar also enhanced the expression of asGM1. Biochemical analysis of selected populations of macrophages indicated that there is also an increase in asGM1 in activated and thioglycollate-elicited macrophages. In addition, there are other major shifts in neutral glycolipids and gangliosides as the cells become activated. Monoclonal antibodies against poly I:C activated and lymphokine-treated macrophages were prepared. We are screening for antibodies that react with specific macrophage markers.